1. Field of the Invention
The present invention relates to a laser device and a microscope, and particularly to a laser device having, in a resonator, a solid laser medium that emits light by receiving excitation light from an excitation unit, and also to a microscope.
2. Description of the Related Art
Conventionally, a solid laser medium to which a transition metal ion such as Ti:S, Cr:BeAl2O4, Cr:LiSAF or the like is added, has a very wide fluorescence band of 100 nm or greater, and has been frequently used as a solid laser medium for a variable wavelength laser or an ultra-short pulse laser.
These solid laser media each having a transition metal ion added thereto have large absorption in a visible wavelength region. The solid laser medium having Ti:S added thereto (a Ti:S laser) is excited by an argon laser (wavelength: 488 nm to 514 nm) or a solid laser that oscillates a second harmonic wave (wavelength: 532 nm) of a fundamental wave of Nd:YVO4 (wavelength: 1064 nm).
However, these excitation light sources each have a large size and a high cost, which in turn becomes a large factor of causing a large-size and high-cost Ti—S laser to be produced. To the contrary, a solid laser medium having a doped Cr3+ ion, such as Cr:BeAl2O4, Cr:LiSAF or the like has a large absorption peak in the wavelength region in the vicinity of 650 nm, and allows excitation with a red semiconductor laser for DVD. For example, U.S. Pat. No. 6,009,114 discloses the technique that a solid laser medium having a transition metal ion added thereto is excited with a semiconductor laser in the region of visible light.
Accordingly, by means of these solid laser media, a small-size and low-cost variable wavelength laser or ultra-short pulse laser can be realized.
However, the limit output of the red semiconductor laser for a DVD is 300 mW or thereabouts in the existing circumstances, and this level of output is sufficient for the laser for a DVD. Therefore, no increase of the output can be expected in the future. For this reason, in the cases of the variable-wavelength laser or ultra-short pulse laser using the solid laser medium having a Cr3+ ion added thereto by means of a red semiconductor laser excitation, there is a limitation to the output of the laser.
The solid laser medium with a Cr3+ ion doped has two or three large absorption peaks in the region of visible wavelength, and also has an absorption peak that exists even in the wavelength region of 500 nm or less. These regions each have the same wavelength of GaN semiconductor laser that has been improved so as to have a high output laser in recent years. Accordingly, by means of the high-output GaN semiconductor laser, high-output variable-wavelength laser or ultra-short pulse laser can be obtained, but such laser devices have not been proposed yet.
For example, Japanese Patent Application Laid-Open (JP-A) No. 2007-281388 discloses a technique that respective polarized lights of excitation light and oscillation light are made to intersect each other, a direction in which excitation light is polarized is set so as to increase light absorption, and a direction in which oscillation light is polarized is set so as to widen a band of fluorescence, thereby allowing achievement of efficient wide-band laser oscillation.
However, in the technique disclosed in JP-A No. 2007-281388 as described above, a semiconductor laser in the region of visible light is not used as an excitation light source, and therefore, the above-described GaN semiconductor laser cannot be used as the excitation light source, and it is difficult to configure a high-output variable-wavelength laser or ultra-short pulse laser.